layer-shell: output property changes precede configure events

Created by: mstoeckl

See commit message for justification. One place where the new guarantee would be helpful is in being able to safely remove a few lines of swaybg (main.c#L196-198), in which the program redraws itself on receiving an output scale change. In practice, this redrawing is not necessary, because the associated configure event always comes afterwards; but this is not guaranteed by the existing spec.

The new text matches the current behavior of kwin and sway when changing output scales.